1. Field of the Invention
The present invention relates to an organic light emitting device.
2. Description of the Related Art
In order to enhance performance capabilities of an organic light emitting device, such as an light emitting efficiency and a life thereof, a number of materials and devices have been developed.
On the other hand, researches for enhancing a light extraction efficiency at which light is extracted outside of an organic light emitting device, and enhancing the performance capabilities such as an light emitting efficiency and a life have also been conducted. For example, it has been reported that a light extraction efficiency is enhanced by providing a layer having a low refractive index formed of silica aerogel or the like in an organic light emitting device (Japanese Patent Application Laid-Open No. 2001-202827).
A light extraction efficiency η at which light generated inside an organic light emitting device is extracted outside thereof depends upon a critical angle θ of a total reflection angle when light is output from a medium having a refractive index of n to the air having a refractive index of 1.00. The critical angle θ is given by the following Expression (1) according to the law of refraction.θ=sin−1(1/n)   (1)
According to the above expression, among the light emitted in the medium having a refractive index of n, only light in an escape cone having an output angle of θ or less can be extracted outside of the organic light emitting device.
Further, the light extraction efficiency η can be given by the following Expression (2).H=1−cosθ≈1/2n2   (2)
In the organic light emitting device illustrated in FIG. 1, light reflection on a side opposite to a light extraction plane does not need to be considered because of the plate structure of a light emitting layer 3, so an output angle θ is determined only by a refractive index n of the light emitting layer 3, and is not influenced at all by the layer structure placed therebetween. Further, it has also been shown that the light extraction efficiency η increases quadratically if the refractive index n of the light emitting layer 3 is small according to Expression (2) (Appl. Phys. Lett., 76, 27 (2000)).
The light emitting layer using an organic light emitting material has a refractive index in a range of 1.7 to 1.8. For example, it is known that Alq3 which is a typical light emitting material has a refractive index of 1.70 (Japanese Patent Application Laid-Open No. H07-240277). In the case where Alq3 is used alone in the light emitting layer, the light extraction efficiency η is determined to be about 17.3% when 1.70 is substituted for n in Expression (2).
As described above, although the light extraction efficiency of an organic light emitting device is determined by the refractive index of a light emitting layer, it has not been reported that the light extraction efficiency of the organic light emitting device is enhanced actually when the refractive index of the light emitting layer is decreased.
On the other hand, an attempt to include a compound containing fluorine in a light emitting layer has been made. For example, it is known that a fluorinated leveling agent is included in a light emitting layer (Japanese Patent Application Laid-Open No. 2002-56980), and a fluorinated surfactant is included in a light emitting layer (Japanese Patent Application Laid-Open No. 2003-272841). It should be noted that the objects thereof are to enhance the flatness of a light emitting layer, suppress coating unevenness, and the like.